Charger for rechargeable batteries

ABSTRACT

A battery charger ( 1 ) for charging rechargeable batteries ( 5 ) and/or battery packs is disclosed. Preferably the charger ( 1 ) can apply two modes of charging a battery. In a normal charging mode a battery is charged to full capacity at a relatively low rate. In a boost charging mode the battery is charged very rapidly and only to maximally 80% of its full capacity. The boost-charging mode makes it possible to provide some charge to the battery ( 5 ) when the time available for charging is limited. Due to the partial charging a much higher charging current than allowed at normal charging may be applied during boost charging.

The present invention relates to a method of charging a rechargeableunit, such as a rechargeable battery or a rechargeable battery pack.

The present invention also relates to a charger for charging arechargeable unit, such as rechargeable battery or a rechargeablebattery pack, said charger comprising a supply unit for supplyingcharging current to a rechargeable unit.

Rechargeable batteries and rechargeable battery packs have a wide spreaduse in the modern life. Many apparatuses, such as mobile phones, batteryoperated electric shavers, battery powered vehicles, electrical toolsetc, are equipped with such batteries.

The rechargeable batteries and battery packs need to be recharged everynow and then. There are several types of chargers that can be used forrecharging rechargeable batteries. A common type of charger employs aconstant current level (CC) throughout the whole charging process of thebattery. Fast chargers of this type employ a high, constant currentuntil the battery is fully charged. An electronic unit in the charger isused to detect end-of-charge and cut off the charging current.

The above-mentioned CC-charger is useful for charging e.g. NiCd(Nickel-Cadmium) and NiMH (Nickel-Metal-Hydride) batteries. With thesebatteries the end-of-charge state can be detected as a sudden increasein the temperature of the battery and as a drop in the terminal voltageof the battery.

Lithium batteries (including lithium-ion, lithium-polymer and lithiumsolid state batteries) cannot be charged by fast chargers of the typementioned above, since lithium batteries do not provide theabove-described indications of end-of-charge and since the maximumvoltage has to be controlled to avoid damage to the lithium batteries.

U.S. Pat. No. 5,994,878 assigned to Ostergaard et al. describes acharger that can handle different types of batteries, including lithiumbatteries. The charger may first charge the battery in a constantcurrent mode and then in a constant voltage mode (constant current thenconstant voltage charging=CCCV). During the first phase of the chargingprocess the charger is in a constant charging current control mode. Thecharging current is controlled at a preset level and the chargingvoltage is monitored. When the charging voltage reaches a certain,preset level the charging process enters a constant charging voltagecontrol mode. In this mode the charging voltage is held substantiallyconstant while the charging current is reduced. The charging process asdescribed in U.S. Pat. No. 5,994,878 is however slow and will not allowquick charging of a battery.

An object of the present invention is to provide a charging method thatmakes it possible to quickly add capacity to rechargeable units.

A further object of the invention is to provide a charger that makes itpossible to quickly add capacity to rechargeable units.

A charging method according to the preamble is characterized in that acharging current corresponding to more than 2 C-rates is supplied to therechargeable unit, and that the supply of charging current isinterrupted before the rechargeable unit has been charged to maximally80% of its full capacity.

It has been found that the interruption of the charging process when therechargeable unit is partially charged makes it possible to increase thecharging current substantially as compared to prior art chargers withoutany risk of damaging the rechargeable unit. The invention thus providesfor very quick partial charging of a rechargeable unit. A typicalsituation where this has very material advantages is when a user who isjust about to leave his home finds out that the battery of e.g. themobile phone or the shaver is empty. By charging just a few minutesaccording to the method described above, the person may obtainsufficient battery charge for his needs in e.g. one day. Another exampleis hybrid electrical vehicles H(EV) and in particular electricalvehicles. A user who finds the batteries of the vehicle empty may in avery short period of time give the batteries a charge that is sufficientfor the ride home.

The measure as defined in claim 2 has the advantage that a rechargeableunit may be fully charged very quickly. The first charging sequence,i.e. charging at a current of more than 2 C-rates to maximally 80% ofthe full capacity, is very rapid. After this sequence has beeninterrupted a second sequence in the form of a normal charging processis started. The normal charging process is slow, but since therechargeable unit was partially charged at a very high rate the overalltime necessary to fully charge the rechargeable unit is considerablyshorter than with prior art charging methods.

The measure as defined in claim 3 has the advantage that extremelyquick, partial charging is possible. Such charging is preferable whenthe charging time is very limited.

The measure as defined in claim 4 has the advantage that a fully oralmost fully charged battery or battery pack is not charged according tothe invention. Thus the risk of damaging the battery is substantiallyeliminated.

A charger according to the preamble is characterized in that the chargerfurther comprises:

-   -   means for supplying a charging current of more than 2 C-rates to        the rechargeable unit; and    -   means for interrupting charging before the rechargeable unit has        been charged to maximally 80% of its full capacity.

The charger described above will provide for very quick partial chargingof a rechargeable unit without the risk of damaging said unit.

The measure as defined in claim 6 has the advantage that the user of thecharger can choose the charging mode that suits the present situation.If the user is in a hurry he chooses boost charging, e.g. by pushing acorresponding button. If there is plenty of time for charging, theperson pushes another button to choose normal charging.

The measure as defined in claim 7 has the advantage that the charger maybe utilized also for fast full charging of a battery. Since normalcharging, in this case charging the battery from partial to fullcapacity, occurs at a low C-rate the battery is not damaged during anypart of the charging process.

The measure as defined in claim 8 has the advantage that the charger canbe used for both partial charging and full charging. After interruptingthe high rate partial charging, the charger automatically shifts to slowrate normal charging to finalize the charging of a battery. The chargercould thus be used both when the user quickly wants some capacity addedto a battery and when the user wants to fully charge the battery. Nocontrol buttons are necessary since the user could terminate thecharging process at any moment in time by just cutting off the supply ofcharging current, e.g. by disconnecting the shaver from the mainssocket.

The measure as defined in claim 9 has the advantage that the userbecomes aware that fast charging is terminated and that the battery ispartially charged and ready for use. The user may then choose tointerrupt the charging process or allow it to proceed in a normalcharging mode.

The measure according to claim 10 provides a simple way of interruptingthe charging process. A timer function is cheap and simple to include ina control unit controlling charging and provides a safe way ofinterrupting the charging process well before the high charging currentcauses any damage to the rechargeable unit. The timer function ispedagogic in that it makes the charging method easy to use andunderstand for the end user.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereafter.

The invention will hereafter be described in more detail and withreference to the appended drawings.

FIG. 1 is a schematic drawing of a charger according to the invention.

FIG. 2 is a diagram showing the charging principles of boost chargingand normal charging.

FIG. 3 is a diagram showing the capacity growth of a battery duringboost charging and during normal charging.

FIG. 4 is a diagram showing the charging times for an empty battery atdifferent initial charging currents and different final depths ofcharge.

The expression C-rate is often used when discussing the charging ofbatteries. 1 C-rate is the charging current that would be needed tocharge an empty battery to its maximum capacity in 1 hour. For eachbattery capacity a certain C-rate means a certain current.

The expression “boost charging” as used in the present application meansa charging method for quickly adding capacity to a battery by chargingit.

The expression “normal charging” as used in the present applicationmeans a charging method for charging, at a rather slow rate, a batteryto its maximum capacity.

The term “cycle life” as used in the present application refers to thenumber of times a battery can be recharged before it has to be disposedof. A long cycle life means that the battery can be recharged manytimes.

In the present application “depth of charge” (DoC) refers to the chargedcapacity of a battery or battery pack. A DoC of 100% means that thebattery has been charged to its maximum capacity.

In FIG. 1 a preferred embodiment of the invention in the form of abattery charger 1 is shown. The battery charger 1 has a charge currentsupply unit 2 adapted to supply a desired voltage and current. Terminalsin the form of electric cables 3, 4 connect the charger 1 to a battery 5that is to be charged. Preferably the cables 3, 4 are each split up intoa current lead and a sense lead for sensing the voltage. The batterycharger 1 has a control unit 6 that controls the current and the voltagesupplied by the supply unit 2 to the battery 5. The control unit 6 isprovided with a selector comprising a first control button,schematically indicated as 7 in FIG. 1, for activating normal chargingof the battery 5. The selector further comprises a second controlbutton, schematically indicated as 8 in FIG. 1, for activating boostcharging of the battery 5.

Normal charging is activated when the user of the charger 1 pushes thenormal charging button 7. Normal charging of the battery 5 is preferablyperformed according to the constant current/constant voltage method(CCCV-method) or at a constant current level (CC-method) depending onthe type of battery to be charged. With the CC-method the current may besupplied in pulses of substantially the same current.

With the CCCV-method, which is often employed for charging lithiumbatteries, the control unit 6 controls the supply unit 2 such that thebattery 5 is first charged in accordance with a constant current mode(CC-mode) while monitoring the voltage (i.e. the voltage as measuredbetween cable 3 and 4). The constant current I_(const) during theCC-mode is typically set low such that an empty battery will obtainabout 50-90% of its nominal max capacity during the CC-mode. A typicalconstant current I_(const) for a lithium battery would be 0.7 C-rate,that is a current that, if held constant during 1 hour, would charge thebattery to 70% of its maximum capacity. When the voltage reaches aftersome time the prescribed maximum voltage V_(max) the control unit 6changes to a constant voltage mode (CV-mode). During the CV-mode thecurrent supplied by the supply unit 2 is controlled such that thevoltage is kept constant at V_(max) while the current is allowed todecrease. The control unit 6 stops the charging process when the currenthas been decreased to a small value or after a predetermined timeinterval that is sufficient for fully charging the battery. The batterythus charged to its maximum capacity in a slow and cautious manner isready for use. The normal charging process provides for a long cyclelife of the battery and a fully charged battery.

With the CC-method, which is often employed for charging NiMH and NiCdbatteries, a constant current level (which may mean a pulsed current) issupplied to the battery throughout the charging process. Charging isinterrupted when a detection method indicates that the battery is fullycharged. One such detection method is temperature measurement. Thetemperature of the battery is measured and when it exceeds a certaintemperature the battery is fully charged. Another detection method ismeasurement of voltage change over time (dV/dt). When a voltage decreaseis detected the battery is fully charged and charging is interrupted.The constant charging current during this type of charging of a NiMHbattery is maximally about 1 C-rate, since a higher charging current maycause oxygen formation in the battery followed by an increased gaspressure. NiCd batteries are charged at maximally 2 C-rates for the samereason. RAM batteries are charged at charging currents below 1 C-rate.

Boost charging of the battery 5 is activated when the user of thecharger 1 pushes the boost charging button 8. Boost charging of thebattery 5 is performed according to the method of the present invention.

In the case of boost charging of lithium batteries the control unit 6controls the supply unit 2 such that a very high initial currentI_(init) is immediately supplied to the battery 5. The control unit 6monitors the voltage supplied (i.e. the voltage as measured betweencable 3 and 4) and controls the current such that the voltage is kept atthe prescribed maximum voltage V_(max). The initial current I_(init) ischosen such that the maximum voltage V_(max) is reached almostimmediately. The control unit 6 will thus control the current suppliedto the battery 5 such that the current is immediately, or after a veryshort period of time, decreased from I_(init) to a lower value. IfI_(init) is very high there will be no constant current phase at all. Ata somewhat lower I_(init), still being very high in relation to thecurrent I_(const) supplied during the CC-mode of the normal chargingprocess, a short period of time may elapse before the current isdecreased. In either case there is no constant current phase of the typedescribed in relation to normal charging.

It has been found that the initial charging current I_(init) applied inthe case of boost charging of lithium batteries should be higher than 1C-rate, i.e. a current that, if held constant, would charge an emptybattery to 50% of its maximum capacity in less than 30 minutes, toprovide quick charging. Initial currents I_(init) higher than 2 C-rates,still more preferably higher than 3,5 C-rates, have been found toprovide a substantial further reduction of the charging time. It hasbeen found that the initial charging current I_(init) should be chosensuch that, at the start of charging, the predetermined maximum chargingvoltage is reached in not more than 2 minutes, since charging during thefirst minutes should be performed at a voltage that is as high aspossible to decrease the time of charging. It has also been found thatthe initial charging current I_(init) should preferably be chosen suchthat the maximum charging voltage is reached in not more than 30seconds, and still more preferably in not more than 5 seconds, toprovide a further substantial reduction of the charging time, chargingduring the first minute being most efficient if performed at a highcurrent and maximum voltage, still without substantial detrimentaleffects on the cycle life.

With other types of batteries, such as NiMH and NiCd, boost charging ispreferably performed at a constant current level, which could be apulsating current or a truly constant current. The charging current ishigher than that allowed in normal charging due to the fact that boostcharging is partial charging. The current in the case of boost chargingof NiCd and NiMH batteries is more than 2 C-rates and more preferablymore than 4 C-rates.

It has been found that boost charging should be interrupted when thebattery 5 has been charged to maximally 80% of its maximum capacity(i.e. 80% DoC) to provide quick charging without substantial negativeeffects on the cycle life. At very high charging currents, such ascharging currents corresponding to 8 C-rates and higher, the charging ispreferably interrupted at a DoC of maximally 60% to avoid damage to thebattery, such as excessive generation of heat or gas in the battery.Such charging would very quickly add capacity to the battery and couldbe used when the user only has a few minutes available. It has furtherbeen found that an interruption of the charging process at a battery DoCof 10-60% provides a relation between time of charging and chargedcapacity that is attractive for most users of the boost chargingfunction. Thus boost charging is preferably used for quick, partialcharging of the battery. To stop boost charging at the proper time forpartial charging, preferably a function for measuring the DoC, i.e. theDoC of the battery at a certain time, is included in the control unit 6.The DoC can be measured by measuring battery parameters according to oneof several methods that are well known to the skilled person. Examplesof such methods of measuring a battery parameter for relating it to theDoC of a battery include open circuit voltage (OCV) measurement andresistance free voltage (RFV) measurement.

The application of boost charging is preferably restricted such that abattery that already has full capacity or almost full capacity cannot besubjected to boost charging. The control unit 6 thus preferably includesa function for measuring the DoC, i.e. the initial DoC, of a presumablyempty battery 5 before any charging, and in particular any boostcharging, may start. To measure the DoC of a battery before starting thecharging process use can be made of one of several methods that are wellknown to the skilled person. Examples of such methods of measuring abattery parameter for relating it to the DoC of a battery includes opencircuit voltage (OCV) measurement, resistance free voltage (RFV)measurement and battery voltage after relaxation (V_(relax)). Whencharging lithium batteries it is also possible to measure the DoC at thevery beginning of the charging process by measuring the time elapsedbefore the charging current starts to decrease, provided that theinitial current I_(init) is chosen such that a short period of timeelapses before the current needs to be decreased to avoid exceeding themaximum charge voltage. The shorter the time before the charging currentis decreased, the higher the initial DoC is. Another alternativeavailable when charging lithium batteries is to measure the slope of thevoltage increase over time when starting boost charging, i.e. measuredV/dt. A large dV/dt then indicates a high initial DoC of the battery.If the measurement of the time elapsed before charging currentdecreases, or of the dV/dt, reveals that the battery already has a highor full capacity, boost charging is immediately interrupted.

In addition to the detrimental effect on the cycle life, the time gainedby boost charging at a high initial DoC is so low that it is preferablyavoided. Boost charging should not be started, or, if in an early phase,stopped immediately, if the battery is found to have an initial DoC ofmore than 70% to avoid detrimental effects on the cycle life. Thecharger 1 may be equipped with a function, such as a flashing light or asound, for indicating that boost charging is interrupted due to highinitial DoC, thus showing the user that the battery already has acertain charge. It has further been found that the relation between timeof charging and charged capacity has a negative effect on the advantagesof starting a boost charging process at an initial DoC of more than 50%.

In a further example of controlling the charging process a timerfunction is provided in the control unit 6. The timer is set to allowboost charging during a certain time, e.g. 5 or 10 minutes, and theninterrupt charging. The timer may be combined with the above describedfunction for avoiding charging at high initial DoC and/or the functionfor interrupting charging at a certain, predetermined DoC. The timerfunction makes the boost charging function easy to use and understandfor the end user.

The control unit may also be adopted to allow boost charging for sometime and then switch to normal charging. In such a case the battery isfirst charged at a high rate for a certain time or to a certain DoC. Thecharger then switches to normal charging and allows charging of thebattery to proceed at a low rate until the battery is fully charged.Preferably an indication, such as the switching on of a light, e.g. aLED, or the sounding of a speaker, is used to indicate that boostcharging is finalized. The user may then choose to interrupt thecharging or allow it to proceed in the normal charging mode for fullycharging the battery at a slow rate.

Boost charging may be applied to all types of rechargeable batteries.Examples of such batteries include nickel metal hydride batteries(NiMH), nickel cadmium batteries (NiCd), lead acid batteries (Pb-acid),rechargeable alkaline manganese batteries (RAM) and lithium batteries.Boost charging has been found to be particularly advantageous forlithium batteries, including lithium ion batteries (Li-ion), lithiumpolymer batteries (Li-polymer), lithium polymer gel batteries(Li-polymer gel) and lithium-metal batteries (Li-metal), since lithiumbatteries must not be charged at high voltages. Due to this fact,chargers for quick charging of lithium batteries did not exist hitherto.

The charger according to the invention may be a stand-alone charger oran integral charger. Thus the charger may be an integral part of anyelectronic or battery driven apparatus. Examples of such an electronicapparatus incorporating a charger are shavers, mobile phones, batterypacks, electrical vehicles, hybrid electrical vehicles H(EV), andpersonal computers. In the case of integral chargers a selector ispreferably located at the housing of the apparatus, such as a shaver, toallow the user to choose the charging mode.

A number of tests were performed to demonstrate the effectiveness of thecharger according to the invention. In the tests a Li-ion battery in theform of a standard Sony US18500 cell with a nominal capacity of 1100 mAhwas used. All tests were performed at 25° C.

FIG. 2 shows the boost charging and normal charging process. The leftvertical axis of FIG. 2 is the charge current I_(charge) in Amperes, theright vertical axis is the charging voltage V_(charge) in Volts and thehorizontal axis is the charged battery capacity in mAh. Normal charging(dotted lines in FIG. 2) takes place at a constant current I_(const) ofabout 1 A until the battery has obtained about 80% of its maximumcapacity. The control unit 6 comprises a charge current limitingfunction which increases the charge current from zero to thepredetermined constant charging current I_(const) and then prevents thecharging current from increasing any further. During this phase ofconstant current (CC) charging the charging voltage increases slowlyfrom 3.6 to 4.2 V, which is the maximum charging voltage of this cell.When the charging voltage reaches 4.2 V the charger switches to constantvoltage mode. Thus the cell is charged with the last 20% of its capacityat a constant voltage of 4.2 V and a decreasing current.

Boost charging is illustrated by means of solid lines in FIG. 2. At thestart of the boost charging process, an initial current I_(init) of 8 Ais supplied to the cell. The charge voltage increases immediately, i.e.in less than 1 second, to the maximum charge voltage of 4.2 V. Thecontrol unit decreases the charging current such that the chargingvoltage is maintained at 4,2 V. The charging current first decreasesrapidly, within 1 minute, to about 4 A. The charging current thendecreases further at a slower rate.

As is indicated in FIG. 2, charging at the end of the chargingprocedure, i.e. the charging of the final 20% of the charging capacity,is similar for normal charging and boost charging. Thus it can beconcluded that the impact of the high initial charging current on thecharge build up is small.

In FIG. 3 the capacity build up as a function of time is shown. Thevertical axis is the charged capacity, i.e. the capacity added to thebattery during charging, in mAh and the horizontal axis is the time inminutes. The maximum charging voltage was 4.2 V. The dotted linedescribes the build up of charge in an empty battery using normalcharging. After normal charging for 10 minutes the DoC of the batteryhas increased to about 16% of its maximum capacity. The constant currentduring the 10 minutes of normal charging was about 1 A corresponding to1 A/1100 mAh=0,9 C-rates. Three tests were carried out with boostcharging using an initial current I_(init) of 8 A corresponding to aninitial C-rate of 8 A/1100 mAh=7,3 C-rates. The results of boostcharging of an empty battery (0% initial DoC) and batteries with 10 and25% initial DoC are shown by means of solid lines in FIG. 3. The emptybattery obtained almost 50% of its maximum capacity after only 10minutes of boost charging. The batteries that had an initial DoC of 10%and 25% respectively showed a somewhat slower capacity build up comparedto the charging of the empty battery. However, as shown in FIG. 3, thecapacity build up at boost charging was in all cases considerablyquicker than capacity build up at normal charging.

In FIG. 4 the impact of the initial charging current I_(init) on thecharging of an empty battery (0% initial DoC) to a certain DoC isdemonstrated. The vertical axis is the initial charging current I_(init)in Amperes and the horizontal axis is the charging time in minutes. Thecurves denote the different DoC, 10-50%, at which charging isinterrupted. Thus the 30% curve represents the time it takes to chargean empty battery to a DoC of 30% of its maximum capacity at differentinitial currents I_(init). The point P represents, by way of example,that, at an initial current I_(init) of 3 A, a DoC of 30% is reachedafter 6.9 minutes.

It is evident from FIG. 4 that an initial charging current I_(init)above 4 A, corresponding to an initial C-rate of about 3.6 C-rates, doesnot further decrease the time required to obtain a certain DoC. On theother hand an initial charging current below 2 A, corresponding to aninitial C-rate of about 1.8 C-rates, results in a substantial increaseof the time required to obtain a certain DoC.

A test was performed at a maximum charging voltage higher than theallowed 4.2 V. The maximum charging voltage was thus set to 4,3 V. Itwas found that an empty battery (0% initial DoC) was charged to a DoC ofalmost 50% at an initial charging current I_(init) of 8 A in 8 minuteswhich is two minutes less than the 10 minutes required at 4.2 V (seeFIG. 3).

Finally, to summarize, a battery charger 1 for charging rechargeablebatteries 5 and/or battery packs is disclosed. Preferably the charger 1can apply two modes of charging a battery. In a normal charging mode abattery is charged to full capacity at a relatively low rate. In a boostcharging mode the battery is charged very rapidly and only to maximally80% of its full capacity. The boost-charging mode makes it possible toprovide some charge to the battery 5 when the time available forcharging is limited. As a result of partial charging, a much highercharging current than that allowed at normal charging may be appliedduring boost charging.

1. A method of charging a rechargeable unit, such as a rechargeablebattery or a rechargeable battery pack, characterized in: that acharging current corresponding to more than 2 C-rates is supplied to therechargeable unit; and that the supply of charging current isinterrupted before the rechargeable unit has been charged to maximum 80%of its full capacity.
 2. A method according to claim 1, wherein saidcharging is followed by normal charging proceeding at a currentcorresponding to maximally 1 C-rate until the rechargeable unit issubstantially fully charged.
 3. A method according to claim 1, wherein acharging current of more than 4 C-rates is used for charging arechargeable unit comprising an NiCd or an NiMH battery.
 4. A methodaccording to claim 1, wherein a measurement of the initial capacity ofthe rechargeable unit is made before charging starts or at the beginningof the charging process, the supply of charging current being stopped ifthe initial capacity is found to be higher than a predetermined initialcapacity.
 5. A charger for charging a rechargeable unit, such as arechargeable battery or a rechargeable battery pack, comprises a supplyunit for supplying charging current to a rechargeable unit,characterized in that the charger further comprises: means for supplyinga charging current of more than 2 C-rates to the rechargeable unit; andmeans for interrupting charging before the rechargeable unit has beencharged to maximally 80% of its full capacity.
 6. A charger according toclaim 5, wherein the charger further comprises a manual selector forchoosing between: a boost charging mode wherein the rechargeable unit ischarged to maximum 80% of its maximum capacity at a currentcorresponding to more than 2 C-rates; and a normal charging mode whereinthe rechargeable unit is fully charged at a current corresponding tomaximum 1 C-rate.
 7. A charger according to claim 6, wherein the chargercomprises means for switching from the boost charging mode to the normalcharging mode when the rechargeable unit has been charged to maximally80% of its full capacity.
 8. A charger according to claim 5, wherein thecharger comprises means for automatically switching to a normal chargingmode for charging the rechargeable unit to full capacity at a currentcorresponding to maximally 1 C-rate after said interruption of saidcharging process.
 9. A charger according to claim 5, wherein the chargercomprises means, such as an LED or a speaker, for providing anindication to the user of the charger that said interruption of saidcharging process has occurred.
 10. A charger according to claim 5,wherein the charger comprises a timer unit, the timer unit being devisedto interrupt said charging process after a predetermined time interval.